The present invention relates to a method of controlling the speed reduction ratio in a continuously variable speed transmission for use in a motor vehicle.
One known method of controlling the speed reduction ratio of a continuously variable speed transmission mounted on a motor vehicle to control the running of the motor vehicle is disclosed in Japanese Laid-Open Patent Publication No. 62(1987)-237164, for example. According to the known control method, a reference engine speed is preset according to an opening of an throttle valve of the engine of the motor vehicle, and the rotational speed of the engine is controlled so as to be equalized with the reference engine speed.
When the accelerator pedal of the motor vehicle is suddenly released to close the throttle valve while the motor vehicle is running at a certain speed reduction ratio, the reference engine speed is also suddenly lowered. Since the actual engine speed is determined by the speed reduction ratio and the vehicle speed, the speed reduction ratio (which is a value of the input speed divided by the output speed) is varied toward a top position (i.e., the speed reduction ratio is decreased).
If the speed reduction ratio is varied too quickly toward the top speed position when the accelerator pedal is suddenly released, the following problem arises: In the event that the driver of the motor vehicle releases and then depresses the accelerator pedal in a short time, since the speed reduction ratio is quickly reduced when the accelerator pedal is released, the power available from the transmission is not large enough when the accelerator pedal is subsequently depressed again, and the driver's feeling with respect to the driving of the motor vehicle is impaired.